It is a braking circuit to be connected in parallel with the power supply.The purpose of the device is to dissipate the regenerative energy which comes from the motor when it is repidly deccelerating and acting as a gneerator.The regenerative energy can be sometimes so high, especially with large motors running on high voltage that it could rise the powersupply voltage to a level when it would damage the drives and even the capacitors and other parts in the power supply. The braking circuit dissipates this energy keeping the voltage down under 180Volts which the DG4S-16035 servo drive can still handle.

Used for high energy short duration dissipation.... Not sure how they would handle repeated use though

RobEinstein ― “If you can't explain it to a six year old, you don't understand it yourself”...working my way through the 1000+ ways things don't work to find the one that doesUC400eth, UC300eth, UCCNC v1.2106, Neuron LiteUCCNC v1.2105 Macro Manual